Blood pressure reactivity differs across the menopause, and estrogen- induced endothelial production of nitric oxide (NO) has been implicated in the regulation of reactivity. In this study, ten premenopausal women (aged 35-49) and ten postmenopausal women (aged 45-57) ate a low nitrate/nitrite (NO metabolite; NOx) for three days, and had reactivity testing on the fourth day in the morning, with two stressors (simulated public speaking and cold presor). Blood samples (resting and post-stress) were analyzed for NOx. Premenopausal women had higher plasma NOx levels at rest and after stress than postmenopausal women. Among the premenopausal women only, a substantial increase in plasma NOx levels during the cold pressor (3 65 5M) was associated with lower reactivity than little or no increase in NOx (p < 0.05). The results of this protocol indicate that NO metabolite levels are higher and are inversely related to reacti vity prior to menopause.